Methods for monitoring and/or controlling the performance of selective catalytic reduction devices

ABSTRACT

Method for controlling and/or monitoring the performance of selective catalytic reduction devices (SCR) and systems incorporating the same are provided. Systems can include a SCR configured to receive reductant at a variable dosing rate, an upstream NOx sensor, and a downstream NOx sensor. Methods can include correlating a SCR reductant dosing signal direction with a SCR downstream NOx signal direction, and determining one or more of NOx breakthrough through the SCR and reductant slip through the SCR. Methods can further comprise identifying reductant slip through the SCR using an upstream NOx sensor signal and a downstream NOx sensor signal. Methods can further comprise comparing an upstream NOx signal to a downstream NOx signal, determining a system objective, and adapting the SCR reductant dosing rate to achieve the system objective in order to identify a faulty upstream NOx sensor.

INTRODUCTION

During a combustion cycle of an internal combustion engine (ICE), air/fuel mixtures are provided to cylinders of the ICE. The air/fuel mixtures are compressed and/or ignited and combusted to provide output torque. After combustion, pistons of the ICE force exhaust gases in the cylinders out through exhaust valve openings and into an exhaust system. The exhaust gas emitted from an ICE, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (CO), unburned hydrocarbons, oxides of nitrogen (NO_(x)), and oxides of sulfur (SO_(x)), as well as condensed phase materials (liquids and solids) that constitute particulate matter. Liquids can include water and hydrocarbons, for example.

Exhaust gas treatment systems may employ catalysts in one or more components configured for accomplishing an after-treatment process such as reducing NO_(x) to produce more tolerable exhaust constituents of nitrogen (N₂) and water (H₂O). One type of exhaust treatment technology for reducing NO_(x) emissions is a selective catalytic reduction device (SCR), which generally includes a substrate or support with a catalyst compound disposed thereon. Passing exhaust over the catalyst converts certain or all exhaust constituents in desired compounds, such as non-regulated exhaust gas components. A reductant is typically sprayed into hot exhaust gases upstream of the SCR, decomposed into ammonia, and absorbed by the SCR. The ammonia then reduces the NO_(x) to nitrogen and water in the presence of the SCR catalyst. Ensuring a suitable reduction of NO_(X) species via an SCR while minimizing reductant slip through a SCR remains a challenge.

SUMMARY

Provided herein are methods for monitoring the performance of a selective catalytic reduction device (SCR) of an exhaust gas treatment system. The exhaust gas treatment system can include a SCR configured to receive reductant at a variable dosing rate and a downstream NOx sensor disposed downstream from the SCR. The method can include correlating a SCR reductant dosing signal direction with a SCR downstream NOx signal direction to determine an operating correlation, and determining one or more of NOx breakthrough through the SCR and reductant slip through the SCR using the operating correlation. The upstream NOx signal is measured by the upstream NOx sensor and the downstream NOx signal is measured by the downstream NOx sensor. A negative operating correlation can indicate a change in NOx breakthrough through the SCR and a positive operating correlation can indicate a change in reductant slip through the SCR. The method can further include determining a reductant loading of the SCR subsequent to correlating. The reductant loading of the SCR can be underloaded while determining a negative operating correlation. The reductant loading of the SCR can be overloaded while determining a positive operating correlation. A positive operating correlation can include an increasing SCR reductant dosing signal direction and an increasing downstream NOx signal direction, and can indicate an increased reductant slip through the SCR. A positive operating correlation can include a decreasing SCR reductant dosing signal direction and a decreasing downstream NOx signal direction, and can indicate a decreased reductant slip through the SCR. A negative operating correlation can include an increasing SCR reductant dosing signal direction and a decreasing downstream NOx signal direction, and can indicate a decreased NOx breakthrough through the SCR. A negative operating correlation can include a decreasing SCR reductant dosing signal direction and an increasing downstream NOx signal direction, and can indicate an increased NOx breakthrough through the SCR. Correlating the SCR reductant dosing signal direction with the SCR downstream NOx signal direction can occur while the SCR is in steady state.

Provided herein are methods for controlling a selective catalytic reduction device (SCR) of an exhaust gas treatment system. The exhaust gas treatment system can include a SCR configured to receive reductant at a variable dosing rate, wherein reductant dosing is controlled by an SCR chemical model to achieve a desired SCR NOx reduction yield by using one or more SCR reductant dosing signals and one or more upstream SCR NOx signals to determine one or more of SCR reductant loading and SCR NOx reduction yield, an upstream NOx sensor disposed upstream from the SCR, and a downstream NOx sensor disposed downstream from the SCR. The method can include determining one or more of a SCR reductant loading and a SCR NOx reduction yield via the SCR chemical model using the upstream NOx signal and the reductant dosing signal, identifying reductant slip through the SCR using the SCR chemical model, adapting the reductant dosing rate to reduce reductant slip through the SCR, subsequently correlating the SCR reductant dosing signal direction with the SCR downstream NOx signal direction to determine an operating correlation, and identifying NOx breakthrough through the SCR using the operating correlation. The upstream NOx signal is measured by the upstream NOx sensor and the downstream NOx signal is measured by the downstream NOx sensor. The reductant dosing signal can include one or more of a measured flow rate of reductant delivered to the SCR, a reductant dosing rate commanded by the SCR chemical model, a reductant dosing mass commanded by the SCR chemical model, and a reductant dosing volume commanded by the SCR chemical model. The SCR reductant dosing signal direction can include a decreasing signal direction, and the SCR downstream NOx signal direction can include an increasing signal direction. The upstream NOx sensor can be faulty and incorrectly sensing a NOx value below the actual NOx process value. The method can further include determining an underloaded reductant loading of the SCR subsequent to correlating the SCR reductant dosing signal direction with the SCR downstream NOx signal direction. The method can further include adapting the reductant dosing rate to reduce NOx breakthrough, subsequent to identifying NOx breakthrough through the SCR. The method can further include identifying the upstream NOx sensor as faulty, subsequent to identifying NOx breakthrough through the SCR. Correlating the SCR reductant dosing signal direction with the SCR downstream NOx signal direction can occur while the SCR is in steady state.

Methods for detecting a faulty sensor of an exhaust gas treatment system are provided. The exhaust gas treatment system can include a selective catalytic reduction device (SCR) configured to receive reductant at a variable dosing rate, an upstream NOx sensor disposed upstream from the SCR, and a downstream NOx sensor disposed downstream from the SCR. The method can include comparing an upstream NOx signal to a downstream NOx signal, determining a system objective, adapting the SCR reductant dosing rate to achieve the system objective, correlating a SCR reductant dosing signal direction to the downstream NOx signal direction to determine if the system objective has been at least partially achieved, and identifying an upstream NOx sensor fault if the system objective has not been at least partially achieved. The upstream NOx signal is measured by the upstream NOx sensor and the downstream NOx signal is measured by the downstream NOx sensor. The method can further include determining one or more of a SCR reductant loading and a SCR NOx reduction yield via an SCR chemical model using the upstream NOx signal and the reductant dosing signal prior to adapting the SCR reductant dosing rate. The system objective can include reducing reductant slip through the SCR, and adapting the SCR reductant dosing comprises lowering the reductant dosing rate. The SCR reductant loading can be below about 10%. The upstream NOx sensor fault can include a measuring a NOx signal below the actual NOx process value. Correlating the SCR reductant dosing signal direction with the SCR downstream NOx signal direction can occur while the SCR is in steady state.

Other objects, advantages and novel features of the exemplary embodiments will become more apparent from the following detailed description of exemplary embodiments and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an schematic view of an exhaust gas treatment system, according to one or more embodiments;

FIG. 2 illustrates a flow chart of a method for monitoring the performance of a selective catalytic reduction device, according to one or more embodiments.

FIG. 3 illustrates a flow chart of a method for controlling a selective catalytic reduction device, according to one or more embodiments.

FIG. 4 illustrates a flow chart of a method for detecting a faulty sensor of an exhaust gas treatment system, according to one or more embodiments.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations.

Generally, this disclosure pertains to the control and monitoring of NOx storage and/or treatment materials, devices, and systems. In particular, this disclosure provides methods for controlling selective catalytic reduction (SCR) devices, and appurtenant NO_(x) sensors, wherein the SCRs are configured to receive exhaust gas streams from an exhaust gas source. As used herein, “NO_(x)” refers to one or more nitrogen oxides. NO_(x) species can include N_(y)O_(x) species, wherein y>0 and x>0. Non-limiting examples of nitrogen oxides can include NO, NO₂, N₂O, N₂O₂, N₂O₃, N₂O₄, and N₂O₅. SCRs are configured to receive reductant, such as at variable dosing rates as will be described below.

In some embodiments, the exhaust gas source generating the exhaust gas streams can be an internal combustion engine (ICE). Methods described herein are germane to ICE systems that can include, but are not limited to, diesel engine systems, gasoline direct injection systems, and homogeneous charge compression ignition engine systems. An ICE can include a plurality of reciprocating pistons attached to a crankshaft which may be operably attached to a driveline, such as a vehicle driveline, to power a vehicle (e.g., deliver tractive torque to the driveline). For example, an ICE can be any engine configuration or application, including various vehicular applications (e.g., automotive, marine and the like), as well as various non-vehicular applications (e.g., pumps, generators and the like). While the ICEs may be described in a vehicular context (e.g., generating torque), other non-vehicular applications are within the scope of this disclosure. Therefore when reference is made to a vehicle, such disclosure should be interpreted as applicable to any application of an ICE.

Moreover, an ICE can generally represent any device capable of generating an exhaust gas stream comprising gaseous (e.g., NO_(x), O₂), carbonaceous, and/or particulate matter species, and the disclosure herein should accordingly be interpreted as applicable to all such devices. As used herein, “exhaust gas” refers to any chemical species or mixture of chemical species which may require treatment, and includes gaseous, liquid, and solid species. For example, an exhaust gas stream may contain a mixture of one or more NO_(x) species, one or more liquid hydrocarbon species, and one more solid particulate species (e.g., ash). It should be further understood that the embodiments disclosed herein may be applicable to treatment of effluent streams not comprising carbonaceous and/or particulate matter species, and, in such instances, ICE can also generally represent any device capable of generating an effluent stream comprising such species. Exhaust gas particulate matter generally includes carbonaceous soot, and other solid and/or liquid carbon-containing species which are germane to ICE exhaust gas or form within an exhaust gas treatment system.

FIG. 1 illustrates an exhaust gas treatment system 10 for treating and/or monitoring the exhaust gas 15 constituents of an ICE 12. System 10 generally includes one or more exhaust gas conduits 14, and one or more exhaust treatment devices. The exhaust gas conduit 14, which can comprise several segments, transports exhaust gas 15 from the ICE 12 to the various exhaust treatment devices of the exhaust gas treatment system 10. In some exemplary embodiments, exhaust gas 15 can comprise NO_(x) species.

In the embodiment as illustrated, the exhaust gas treatment device 26 comprises a SCR. SCRs can include selective catalytic filter (SCRF) devices which provide the catalytic aspects of SCRs in addition to particulate filtering capabilities. Upstream and downstream are defined in relation to the direction of the flow of exhaust gas 15 from ICE 12. As shown in FIG. 1, SCR 26 includes an upstream side 26′ and a downstream side 26″. As can be appreciated, system 10 of the present disclosure can include various additional treatment devices, including oxidation catalyst devices (not shown), and particulate filter devices (not shown), among others. SCR 26 is configured to receive reductant, such as at variable dosing rates. Reductant 36 can be supplied from a reductant supply source (not shown) and injected into the exhaust gas conduit 14 at a location upstream of the SCR 26 using an injector 46, or other suitable method of delivery. Reductant 36 can be in the form of a gas, a liquid, or an aqueous solution, such as an aqueous urea solution. Reductant 36 can be mixed with air in the injector 46 to aid in the dispersion of the injected spray. A turbulator 48 (i.e., mixer) can also be disposed within the exhaust conduit 14 in close proximity to the injector 46 and/or the SCR 26 to further assist in thorough mixing of reductant 36 with the exhaust gas 15 and/or even distribution throughout the SCR 26.

System 10 can further include a control module 50 operably connected via a number of sensors to monitor the engine 12 and/or the exhaust gas treatment system 10. As used herein, the term module refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. For example, module 50 can execute a SCR chemical model, as described below. The control module 50 can be operably connected to ICE 12, SCR 26, and/or one or more sensors. As shown, control module 50 is in communication upstream NO_(x) sensor 60 and downstream NO_(x) sensor 62, disposed downstream of SCR 26, each of which are in fluid communication with exhaust gas conduit 14. Upstream NO_(x) sensor 60 is disposed downstream of the ICE 12 and upstream of both SCR 26 and turbulator 48. Upstream NO_(x) sensor 60 and downstream NO_(x) sensor 62 are configured to detect a NO_(x) level proximate their location within exhaust gas conduit 14, and generate a NOx signal which corresponds to the NOx level. A NOx level can comprise a concentration, a mass flow rate, or a volumetric flow rate, in some embodiments. A NOx signal generated by a NOx sensor can be interpreted by control module 50, for example. Control module 50 can optionally be in communication one or more temperature sensors, such as upstream temperature sensor 52, disposed upstream from SCR 26, and downstream temperature sensor 54, disposed downstream of SCR 26.

In general, the SCR 26 includes all devices which utilize a reductant 36 and a catalyst to NO and NO₂ to harmless components. The SCR 26 can include, for example, a flow-through ceramic or metal monolith substrate that can be packaged in a shell or canister having an inlet and an outlet in fluid communication with the exhaust gas conduit 14 and optionally other exhaust treatment devices. The shell or canister can ideally comprise a substantially inert material, relative to the exhaust gas constituents, such as stainless steel. The substrate can include a SCR catalyst composition applied thereto.

The substrate body can, for example, be a ceramic brick, a plate structure, or any other suitable structure such as a monolithic honeycomb structure that includes several hundred to several thousand parallel flow-through cells per square inch, although other configurations are suitable. Each of the flow-through cells can be defined by a wall surface on which the SCR catalyst composition can be washcoated. The substrate body can be formed from a material capable of withstanding the temperatures and chemical environment associated with the exhaust gas 15. Some specific examples of materials that can be used include ceramics such as extruded cordierite, α-alumina, silicon carbide, silicon nitride, zirconia, mullite, spodumene, alumina-silica-magnesia, zirconium silicate, sillimanite, petalite, or a heat and corrosion resistant metal such as titanium or stainless steel. The substrate can comprise a non-sulfating TiO₂ material, for example. The substrate body can be a PF device, as will be discussed below.

The SCR 26 catalyst composition is generally a porous and high surface area material which can operate efficiently to convert NO_(x) constituents in the exhaust gas 15 in the presence of a reductant 36, such as ammonia. For example, the catalyst composition can contain a zeolite impregnated with one or more base metal components such as iron (Fe), cobalt (Co), copper (Cu), vanadium (V), sodium (Na), barium (Ba), titanium (Ti), tungsten (W), and combinations thereof. In a particular embodiment, the catalyst composition can contain a zeolite impregnated with one or more of copper, iron, or vanadium. In some embodiments the zeolite can be a β-type zeolite, a Y-type zeolite, a ZM5 zeolite, or any other crystalline zeolite structure such as a Chabazite or a USY (ultra-stable Y-type) zeolite. In a particular embodiment, the zeolite comprises Chabazite. In a particular embodiment, the zeolite comprises SSZ. Suitable SCR catalyst compositions can have high thermal structural stability, particularly when used in tandem with particulate filter (PF) devices or when incorporated into SCRF devices, which are regenerated via high temperature exhaust soot burning techniques.

The SCR catalyst composition can optionally further comprise one or more base metal oxides as promoters to further decrease the SO₃ formation and to extend catalyst life. The one or more base metal oxides can include WO₃, Al₂O₃, and MoO₃, in some embodiments. In one embodiment, WO₃, Al₂O₃, and MoO₃ can be used in combination with V₂O₅.

The SCR 26 generally uses a reductant 36 to reduce NO_(x) species (e.g., NO and NO₂) to harmless components. Harmless components include one or more of species which are not NO_(x) species, such as diatomic nitrogen, nitrogen-containing inert species, or species which are considered acceptable emissions, for example. The reductant 36 can be ammonia (NH₃), such as anhydrous ammonia or aqueous ammonia, or generated from a nitrogen and hydrogen rich substance such as urea (CO(NH₂)₂). Additionally or alternatively, the reductant 36 can be any compound capable of decomposing or reacting in the presence of exhaust gas 15 and/or heat to form ammonia. Equations (1)-(5) provide exemplary chemical reactions for NO_(x) reduction involving ammonia.

6NO+4NH₃→5N₂+6H₂O  (1)

4NO+4NH₃+O₂→4N₂+6H₂O  (2)

6NO₂+8NH₃→7N₂+12H₂O  (3)

2NO₂+4NH₃+O₂→3N₂+6H₂O  (4)

NO+NO₂+2NH₃→2N₂+3H₂O  (5)

It should be appreciated that Equations (1)-(5) are merely illustrative, and are not meant to confine the SCR 26 to a particular NO_(x) reduction mechanism or mechanisms, nor preclude the operation of other mechanisms. The SCR 26 can be configured to perform any one of the above NO_(x) reduction reactions, combinations of the above NO_(x) reduction reactions, and other NO_(x) reduction reactions.

The reductant 36 can be diluted with water in various implementations. In implementations where the reductant 36 is diluted with water, heat (e.g., from the exhaust) evaporates the water, and ammonia is supplied to the SCR 26. Non-ammonia reductants can be used as a full or partial alternative to ammonia as desired. In implementations where the reductant 36 includes urea, the urea reacts with the exhaust to produce ammonia, and ammonia is supplied to the SCR 26. Reaction (6) below provides an exemplary chemical reaction of ammonia production via urea decomposition.

CO(NH₂)₂+H₂O→2NH₃+CO₂  (6)

It should be appreciated that Equation (6) is merely illustrative, and is not meant to confine the urea or other reductant 36 decomposition to a particular single mechanism, nor preclude the operation of other mechanisms.

The SCR 26 can store (i.e., absorb, and/or adsorb) reductant for interaction with exhaust gas 15. For example, the reductant can be stored within the SCR as ammonia. A given SCR has a reductant capacity, or an amount of reductant or reductant derivative it is capable of storing. The amount of reductant stored within a SCR relative to the SCR capacity can be referred to as the SCR “reductant loading”, and can be indicated as a % loading (e.g., 90% reductant loading) in some instances. During operation of SCR 26, injected reductant 36 as stored in the SCR and consumed during reduction reactions with NOx species and must be continually replenished. Determining the precise amount of reductant 36 to inject is critical to maintaining exhaust gas 15 emissions at acceptable levels: insufficient reductant levels with the system 10 (e.g., within SCR 26) can result in undesirable NOx species emissions (“NOx breakthrough”) from the system (e.g., via a vehicle tailpipe), while excessive reductant 36 injection can result in undesirable amounts of reductant 36 passing through the SCR 26 unreacted or exits the SCR as an undesired reaction product (“reductant slip”). Reductant slip and NOx breakthrough can also occur when the SCR is below a “light-off” temperature. SCR 26 dosing logic can be utilized to command reductant 36 dosing, and adaptations thereof, and can be implemented by module 50, for example.

A reductant 36 injection dosing rate (e.g., grams per second) can be determined by a SCR chemical model which predicts the amount of reductant 36 stored in the SCR 26 based on signals from one or more of reductant 36 injection (e.g., feedback from injector 46) and upstream NOx (e.g., NOx signal from upstream NOx sensor 60). The SCR chemical model further predicts NOx levels of exhaust gas 15 discharged from the SCR 26. The SCR chemical model can be implemented by module 50. The SCR chemical model can be updatable by one or more process values over time, for example. A dosing governor (not shown), such as one controlled by module 50, monitors the reductant 36 storage level predicted by the SCR chemical model, and compares the same to a desired reductant 36 storage level. Deviations between the predicted reductant 36 storage level and the desired reductant 36 storage level can be continuously monitored and will trigger a dosing adaptation to increase or decrease reductant 36 dosing in order to eliminate or reduce the deviation. For example, the reductant 36 dosing rate can be adapted to achieve a desired NO_(x) concentration or flow rate in exhaust gas 15 downstream of the SCR 26, or achieve a desired SCR 26 NO_(x) conversion rate. A desired conversion rate can be determined by many factors, such as the characteristics of SCR 26 (e.g., catalyst type) and/or operating conditions of the system (e.g., ICE 12 operating parameters).

Over time, inaccuracies of the SCR chemical model can compound to appreciative errors between modeled SCR reductant loading and actual loading. Accordingly, the SCR chemical model can be continuously corrected to minimize or eliminate errors. One method for correcting an SCR chemical model includes comparing the modeled SCR discharge exhaust gas NOx levels to the actual NOx levels (e.g., as measured by downstream NOx sensor 62) to determine a discrepancy, and subsequently correcting the model to eliminate or reduce the discrepancy. Because NOx sensors (e.g., downstream NOx sensor 62) are cross-sensitive to reductant (e.g., NH₃) and NOx, it is critical to distinguish between reductant signals and NOx signals as reductant slip can be confused with insufficient NOx conversion. In such instances, SCR 26 control logic can command an increased reductant dosing, thereby exacerbating reductant slip in the attempt to mitigate NO_(x) breakthrough.

One passive analysis technique used to distinguish between reductant signals and NOx signals is a correlation method which includes comparing the upstream NOx concentration (e.g., such as measured by upstream NOx sensor 60) movement with the downstream NOx concentration (e.g., such as measured by downstream NOx sensor 62), wherein diverging concentration directions can indicate an increase or decrease in reductant 36 slip. For example, if the upstream NOx concentration decreases and downstream NOx concentration increases, reductant 36 slip can be identified as increasing. Similarly, if the upstream NOx concentration increases and downstream NOx concentration decreases, reductant 36 slip can be identified as decreasing. A second passive analysis technique used to distinguish between reductant signals and NOx signals is a frequency analysis. NOx signals generated by NOx sensors can include multiple frequency components (e.g., high frequency and low frequency) due to the variation of the NOx and reductant 36 concentrations during transient conditions. High frequency signals generally relate only to NOx concentration, while low frequency signals generally relate to both NOx concentration and reductant 36 concentration. High frequency signals for upstream NOx and downstream NOx are isolated and used to calculate a SCR NOx conversion ratio, which is then applied to the isolated low pass upstream NOx signal to determine a low frequency downstream NOx signal. The calculated low frequency downstream NOx signal is then compared to the actual isolated low frequency downstream NOx signal, wherein a deviation between the two values can indicate reductant 36 slip.

One drawback of passive analysis techniques such as the correlation method and frequency method described above is that they rely on the proper operation of two NOx sensors. For example, a faulty upstream NOx sensor (e.g., upstream NOx sensor 60) can generate a NOx signal which is lower than the actual NOx level proximate the upstream NOx sensor causing the SCR chemical model to predict higher reductant storage than the actual storage. Accordingly, NOx breakthrough would be incorrectly identified as reductant slip, and reductant dosing would be commanded such that NOx breakthrough would be exacerbated (i.e., reductant dosing would be decreased). Further, the SCR chemical model would be updated using the inaccurate upstream NOx measurement, and the exacerbated NOx breakthrough would endure.

Another drawback of the correlation and frequency passive analysis techniques is that they cannot be implemented while the SCR is in steady state. “Steady state” is determined, for example, by taking the root mean square value of a NOx signal upstream from SCR 26 (e.g., such as measured by upstream NOx sensor 60) over a moving time frame; a sufficiently small value indicates a minimal variation in upstream NOx concentration and the SCR can be considered to be in steady state. For example, a steady state condition can be comprise a root mean square value of the upstream NOx concentration of less than about 30 ppm, less than about 20 ppm, or less than about 10 ppm. SCR steady state conditions can often correlate with ICE 12 steady state conditions (e.g., generally consistent RPM, fuel injection, temperature, etc.) Intrusive tests can be used to distinguish between reductant signals and NOx signals, which includes halting all or most reductant dosing for a period of time. While intrusive tests can be performed under steady state conditions, they can, in some circumstances, yield undesirable exhaust emissions during the test period, such as emissions with an increased NOx concentration.

Provided herein are methods for monitoring and/or controlling SCRs which utilize reductant dosing levels and downstream NOx levels to eliminate or reduce the above issues with NOx/reductant cross-sensitivity and upstream NOx sensor faults. Advantageously, the methods described below can be implemented during steady state SCR conditions. The methods can be utilized in combination with traditional methods which utilize SCR upstream NOx levels and downstream NOx levels. The methods will be described in reference to system 10, but are not to be construed as limited by the particular configuration of system 10 as described.

FIG. 2 illustrates a method 200 for monitoring the performance of a SCR 26 of an exhaust gas treatment system 10 comprising correlating 220 a SCR 26 reductant 36 dosing signal direction with a SCR 26 downstream NOx signal direction to determine an operating correlation, and determining 230 one or more of NOx breakthrough through the SCR and reductant slip through the SCR using the operating correlation. Determining the operating correlations allows for the downstream NOx signal to be dissected into separate NOx signals and reductant (e.g., NH₃) signals. The reductant dosing signal can comprise one or more of a measured flow rate of reductant delivered to the SCR, a commanded reductant dosing rate, a commanded reductant dosing mass, and a commanded reductant dosing volume, for example. A negative operating correlation indicates a change in NOx breakthrough through the SCR and a positive operating correlation indicates a change in reductant slip through the SCR. Such negative and positive correlations can be used to dissect the downstream NOx signal into NOx and reductant (e.g., NH₃) components. Method 200 is particularly suitable for use with systems, such as system 10, which include downstream NOx sensors having cross-sensitivity to reductant (e.g., NH₃) and NOx. The correlated 220 reductant 36 dosing signals and downstream NOx signals can measured during a dosing adaptation, in some embodiments. Method 200 can be implemented during steady state conditions. In some embodiments described below, the operating correlation determined by correlating 220 the SCR 26 reductant 36 dosing signal direction with the SCR 26 downstream NOx signal direction can be used to determined SCR 26 reductant loading. Accordingly, method 200 can optionally further include determining 225 a reductant loading of the SCR 26 subsequent to correlating 220 the SCR 26 reductant 36 dosing signal direction with the SCR 26 downstream NOx signal direction.

In one embodiment, a positive operating correlation comprises an increasing SCR 26 reductant 36 dosing signal direction and an increasing downstream NOx signal direction, and indicates an increased reductant 36 slip through the SCR. In some such embodiments, the SCR reductant loading can be overloaded. As used herein, “overloaded” refers to a SCR reductant loading of greater than about 50%, greater than about 55%, or greater than about 60% of the desired reductant loading. In other such an embodiments, the reductant loading of the SCR 26 can be greater than about 85%, greater than about 90%, or greater than about 95%.

In one embodiment, a positive operating correlation comprises a decreasing SCR 26 reductant 36 dosing signal direction and a decreasing downstream NOx signal direction, and indicates a decreased reductant slip through the SCR. In some such embodiments, the SCR reductant loading can be overloaded. In other such an embodiments, the reductant loading of the SCR 26 can be greater than about 85%, greater than about 90%, or greater than about 95%.

In one embodiment, a negative operating correlation comprises an increasing SCR 26 reductant 36 dosing signal direction and a decreasing downstream NOx signal direction, and indicates a decreased NOx breakthrough through the SCR 26. In some such embodiments, the SCR reductant loading can be underloaded. As used herein, “underloaded” refers to a SCR reductant loading of less than about 50%, less than about 45%, or less than about 40% of the desired reductant loading. In other such an embodiments, the reductant loading of the SCR 26 can be less than about 15%, less than about 10%, or less than about 5%.

In one embodiment, a negative operating correlation comprises a decreasing SCR 26 reductant 36 dosing signal direction and an increasing downstream NOx signal direction, and indicates an increased NOx breakthrough through the SCR. In some such embodiments, the SCR reductant loading can be underloaded. In other such an embodiments, the reductant loading of the SCR 26 can be less than about 15%, less than about 10%, or less than about 5%.

FIG. 3 illustrates method 300 for controlling a SCR 26 of an exhaust gas treatment system, such as system 10, wherein the system includes a SCR 26 configured to receive reductant at a variable dosing rate, wherein reductant dosing is controlled by an SCR 26 chemical model to achieve a desired SCR 26 NOx reduction by using one or more SCR reductant dosing signals and one or more upstream SCR 26 NOx signals to determine one or more of SCR 26 reductant loading and SCR NOx reduction, an upstream NOx sensor 60 disposed upstream from the SCR 26, and a downstream NOx sensor 62 disposed downstream from the SCR. Method 300 comprises determining 310 one or more of a SCR reductant loading and a SCR 26 NOx reduction yield via the SCR chemical model using the upstream NOx signal and the reductant dosing signal, identifying 320 reductant slip through the SCR using the SCR 26 chemical model, adapting 330 the reductant 36 dosing rate to reduce reductant 36 slip through the SCR 26, subsequently correlating 340 a SCR 26 reductant 36 dosing signal direction to a SCR 26 downstream NOx signal direction to determine an operating correlation, and identifying 350 NOx breakthrough through the SCR 26 using the operating correlation. Determining 310, identifying 320, and adapting 330 represent the normal operation of a SCR 26, upon which this disclosure offers improvements. In some embodiments, the upstream NOx sensor 60 is incorrectly sensing a NOx value below the actual NOx process value, and accordingly the SCR 26 control logic improperly infers the occurrence of reductant 36 slip. Subsequently comparing 340 a SCR 26 reductant 36 dosing signal to a SCR 26 downstream NOx signal allows for the downstream NOx signal to be dissected into separate NOx signals and reductant (e.g., NH₃) signals, which further allows for the SCR 26 control logic to properly identify an increased NOx breakthrough.

In some embodiments, adapting 330 the reductant dosing rate can comprise reducing the reductant dosing rate, and the SCR reductant dosing signal direction can comprise a decreasing signal direction and the SCR downstream NOx signal direction can comprise an increasing signal direction. In some embodiments, the operating correlation determined by correlating 340 the SCR 26 reductant 36 dosing signal direction with the SCR 26 downstream NOx signal direction can be used to determined SCR 26 reductant loading. Accordingly, method 300 can optionally further include determining 345 a reductant loading of the SCR 26 subsequent to correlating 240 the SCR 26 reductant 36 dosing signal direction with the SCR 26 downstream NOx signal direction. In embodiments where NOx breakthrough through the SCR 26 is identified 350, the SCR can be underloaded. In other such an embodiments, the reductant loading of the SCR 26 can be less than about 15%, less than about 10%, or less than about 5%. Method 300 can be implemented during steady state conditions. Specifically, correlating the SCR reductant dosing signal direction with the SCR downstream NOx signal direction can occur while the SCR is in steady state.

Method 300 can further optionally comprise one or more of adapting 360 the reductant 36 dosing rate to reduce NOx breakthrough, and identifying 370 the upstream NOx sensor 60 as faulty. In some embodiments, the reductant loading of the SCR 26 can be less than about 15%, less than about 10%, or less than about 5%. In such an embodiment, the reductant within the SCR 26 can alternatively be nearly all consumed.

FIG. 4 illustrates method 400 for detecting a faulty sensor of an exhaust gas treatment system. The system includes a SCR 26 configured to receive reductant at a variable dosing rate, an upstream NOx sensor 60, and a downstream NOx sensor 62 as illustrated in FIG. 1. Method 400 comprises determining 410 one or more of a SCR reductant loading and a SCR 26 NOx reduction yield via an SCR chemical model using the upstream NOx signal and the reductant dosing signal, comparing 420 an upstream NOx signal to a downstream NOx signal, determining 430 a system objective, adapting 440 the SCR reductant 36 dosing rate to achieve the system objective, correlating 450 a SCR reductant dosing signal direction to the downstream NOx signal direction to determine if the system objective has been at least partially achieved, and identifying 460 an upstream NOx sensor fault if the system objective has not been at least partially achieved. In some embodiments, determining 410 is optional. Method 400 can be implemented during steady state conditions.

In some instances, a faulty upstream NOx sensor 60 improperly detecting a NOx level below the actual NOx process level, when compared 420 with the downstream NOx sensor 62 signal, may cause SCR 26 dosing logic to improperly infer unsuitable reductant 36 slip through the SCR 26. Accordingly, determining 420 a system object would comprise reducing reductant 36 slip, and adapting 440 reductant 36 dosing would comprise reducing reductant 36 dosing. Subsequent to adapting 440 the reductant 36 dosing, correlating 450 the SCR 26 reductant 36 dosing signal direction to the downstream NOx signal direction would indicate an increasing downstream NOx signal (i.e., NOx breakthrough). Method 400 accordingly is able to diagnose the upstream NOx sensor 60 as faulty. Particularly, method 400 is able to diagnose the upstream NOx sensor 60 as incorrectly detecting a NOx value below the actual process value. In some embodiments, the reductant 36 loading of the SCR 26 can be less than about 15%, less than about 10%, or less than about 5%. In such an embodiment, the reductant 36 within the SCR 26 can alternatively be nearly all consumed.

In some instances, a faulty upstream NOx sensor 60 improperly detecting a NOx level above the actual NOx process level, when compared 420 with the downstream NOx sensor 62 signal, may cause SCR 26 dosing logic to improperly infer an increased NOx concentration upstream of the SCR 26. Accordingly, determining 420 a system object would comprise reducing an increased amount of NOx species, and adapting 440 reductant 36 dosing would comprise increasing reductant 36 dosing. Subsequent to adapting 440 the reductant 36 dosing, correlating 450 the SCR 26 reductant 36 dosing signal direction to the downstream NOx signal direction would indicate an increasing downstream NOx signal (i.e., reductant slip). Method 400 accordingly is able to diagnose the upstream NOx sensor 60 as faulty. Particularly, method 400 is able to diagnose the upstream NOx sensor 60 as incorrectly detecting a NOx value above the actual process value. In some embodiments, the SCR is overloaded. In some embodiments, the reductant 36 loading of the SCR 26 can be at least about 85%, at least about 90%, or at least about 95%. In such an embodiment, the SCR 26 can be nearly fully loaded with reductant 36. Method 400 can be implemented during steady state conditions. Specifically, correlating the SCR reductant dosing signal direction with the SCR downstream NOx signal direction can occur while the SCR is in steady state.

While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes can include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and can be desirable for particular applications. 

What is claimed is:
 1. A method for monitoring the performance of a selective catalytic reduction device (SCR) of an exhaust gas treatment system, wherein the system includes a SCR configured to receive reductant at a variable dosing rate and a downstream NO_(x) sensor disposed downstream from the SCR, the method comprising: correlating a SCR reductant dosing signal direction with a SCR downstream NO_(x) signal direction to determine an operating correlation; and determining one or more of NO_(x) breakthrough through the SCR and reductant slip through the SCR using the operating correlation; wherein a negative operating correlation indicates a change in NO_(x) breakthrough through the SCR and a positive operating correlation indicates a change in reductant slip through the SCR, and the upstream NO_(x) signal is measured by the upstream NO_(x) sensor and the downstream NO_(x) signal is measured by the downstream NO_(x) sensor.
 2. The method of claim 1, further comprising determining a reductant loading of the SCR subsequent to correlating, wherein a reductant loading of the SCR is underloaded while determining a negative operating correlation and a reductant loading of the SCR is overloaded while determining a positive operating correlation.
 3. The method of claim 1, wherein a positive operating correlation comprises an increasing SCR reductant dosing signal direction and an increasing downstream NO_(x) signal direction, and indicates an increased reductant slip through the SCR.
 4. The method of claim 1, wherein a positive operating correlation comprises a decreasing SCR reductant dosing signal direction and a decreasing downstream NO_(x) signal direction, and indicates a decreased reductant slip through the SCR.
 5. The method of claim 1, wherein a negative operating correlation comprises an increasing SCR reductant dosing signal direction and a decreasing downstream NO_(x) signal direction, and indicates a decreased NO_(x) breakthrough through the SCR.
 6. The method of claim 1, wherein a negative operating correlation comprises a decreasing SCR reductant dosing signal direction and an increasing downstream NO_(x) signal direction, and indicates an increased NO_(x) breakthrough through the SCR.
 7. The method of claim 1, wherein correlating the SCR reductant dosing signal direction with the SCR downstream NO_(x) signal direction occurs while the SCR is in steady state.
 8. A method for controlling a selective catalytic reduction device (SCR) of an exhaust gas treatment system, wherein the system includes a SCR configured to receive reductant at a variable dosing rate, wherein reductant dosing is controlled by an SCR chemical model to achieve a desired SCR NO_(x) reduction yield by using one or more SCR reductant dosing signals and one or more upstream SCR NO_(x) signals to determine one or more of SCR reductant loading and SCR NO_(x) reduction yield, an upstream NO_(x) sensor disposed upstream from the SCR, and a downstream NO_(x) sensor disposed downstream from the SCR, the method comprising: determining one or more of a SCR reductant loading and a SCR NO_(x) reduction yield via the SCR chemical model using the upstream NO_(x) signal and the reductant dosing signal; identifying reductant slip through the SCR using the SCR chemical model; adapting the reductant dosing rate to reduce reductant slip through the SCR; subsequently correlating the SCR reductant dosing signal direction with the SCR downstream NO_(x) signal direction to determine an operating correlation; and identifying NO_(x) breakthrough through the SCR using the operating correlation; wherein the upstream NO_(x) signal is measured by the upstream NO_(x) sensor, the downstream NO_(x) signal is measured by the downstream NO_(x) sensor, and the reductant dosing signal comprises one or more of a measured flow rate of reductant delivered to the SCR, a reductant dosing rate commanded by the SCR chemical model, a reductant dosing mass commanded by the SCR chemical model, and a reductant dosing volume commanded by the SCR chemical model.
 9. The method of claim 8, wherein the SCR reductant dosing signal direction comprises a decreasing signal direction, and the SCR downstream NO_(x) signal direction comprises an increasing signal direction.
 10. The method of claim 8, wherein the upstream NO_(x) sensor is faulty and incorrectly sensing a NO_(x) value below the actual NO_(x) process value.
 11. The method of claim 8, further comprising determining an underloaded reductant loading of the SCR subsequent to correlating the SCR reductant dosing signal direction with the SCR downstream NO_(x) signal direction.
 12. The method of claim 8, further comprising adapting the reductant dosing rate to reduce NO_(x) breakthrough, subsequent to identifying NO_(x) breakthrough through the SCR.
 13. The method of claim 8, further comprising identifying the upstream NO_(x) sensor as faulty, subsequent to identifying NO_(x) breakthrough through the SCR.
 14. The method of claim 8, wherein correlating the SCR reductant dosing signal direction with the SCR downstream NO_(x) signal direction occurs while the SCR is in steady state.
 15. A method for detecting a faulty sensor of an exhaust gas treatment system, wherein the system includes a selective catalytic reduction device (SCR) configured to receive reductant at a variable dosing rate, an upstream NO_(x) sensor disposed upstream from the SCR, and a downstream NO_(x) sensor disposed downstream from the SCR, the method comprising: comparing an upstream NO_(x) signal to a downstream NO_(x) signal; determining a system objective; adapting the SCR reductant dosing rate to achieve the system objective; correlating a SCR reductant dosing signal direction to the downstream NO_(x) signal direction to determine if the system objective has been at least partially achieved; and identifying an upstream NO_(x) sensor fault if the system objective has not been at least partially achieved; wherein the upstream NO_(x) signal is measured by the upstream NO_(x) sensor and the downstream NO_(x) signal is measured by the downstream NO_(x) sensor.
 16. The method of claim 15, further comprising determining one or more of a SCR reductant loading and a SCR NO_(x) reduction yield via an SCR chemical model using the upstream NO_(x) signal and the reductant dosing signal prior to adapting the SCR reductant dosing rate.
 17. The method of 15, wherein the system objective comprises reducing reductant slip through the SCR, and adapting the SCR reductant dosing comprises lowering the reductant dosing rate.
 18. The method of 17, wherein the SCR reductant loading is below about 10%.
 19. The method of 17, wherein the upstream NO_(x) sensor fault comprises a measuring a NO_(x) signal below the actual NO_(x) process value.
 20. The method of claim 15, wherein correlating the SCR reductant dosing signal direction with the SCR downstream NO_(x) signal direction occurs while the SCR is in steady state. 